Life Detection Ladder

The direct detection of extant life has not been attempted by NASA since the Viking Missions in the late 1970s. NASA’s Ladder of Life Detection was generated to stimulate and support discussions among scientists and engineers about how one would detect extant life beyond Earth but within our Solar System (particularly on Europa and the other “Ocean Worlds”). In creating the Ladder, we started with the NASA definition of life, “Life is a self sustaining chemical system capable of Darwinian evolution” and considered the specific features of the one life we know —Terran life.

Table: Life Detection Ladder

Ladder Rung

Feature

Measurement

Instrument

Target

Likelihood

Specific to Earth Life vs. Potential for Generic Life

Ambiguity of Feature

Ambiguity of Interpretation

False Positive

False Negative

Detectability

Life (metabolism, growth, reproduction)

Darwinian Evolution

changes in heritable traits in response to selective pressures

not possible

no

~

~

~

~

Growth and Reproduction

concurrent life stages or identifiable reproductive form [growth and reproduction]

Column Defintion
Likelihood - A summary of the factors to the right including specificty, abiguity, false postive, false negative and detectability.
Functional Molecules - Molecules that are almost certainly produced biologically and are produced because they serve catalytic, metabolic, structural, protective or energy harvesting functions.
False Positive - A positive determination based on detection due to contamination or a postive detection caused by an unintended interference with the method of detection.
False Negative - Lack of detection when the factor is actually present either in the sampe or the system at large. For example, low signal to background or signal below the limit of detection.

The rungs of the Ladder were assembled from features that can be used to access (1) potential habitability, (2) suspicious biomaterials that could be biogenic or abiogenic, and (3) active processes of life. The lowest rungs are the least directly related to extant life and in some cases are the easiest to measure. For each rung (feature), the target and potential flight instruments for measurement were identified. Our ability to detect and properly interpret a measurement was evaluated in terms of how specific the feature was for Terran-type life, how likely the feature could be produced abiotically (called ambiguity), how likely the measurement would be a false positive due to contamination or measurement interference, how likely the measurement would be a false negative (missing life when it is present), and easy the measurement is to make (detectability). Download this excel spreadsheet here.

For generations the definition of life has eluded scientists and philosophers. (Many have come to recognize that the concept of “definition” itself is difficult to define) We can, however, list characteristics of the one example of life that we know—life on Earth:

• It is chemical in essence; terran living systems contain molecular species that undergo chemical transformations (metabolism) under the direction of molecules (enzyme catalysts) whose structures are inherited, and heritable information is itself carried by molecules.
• To have directed chemical transformations, terran living systems exploit a thermodynamic disequilibrium.
• The biomolecules that terran life uses to support metabolism, build structures, manage energy, and transfer information take advantage of the covalent bonding properties of carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur and the ability of heteroatoms, primarily oxygen and nitrogen, to modulate the reactivity of hydrocarbons.
• Terran biomolecules interact with water to be soluble (or not) or to react (or not) in a way that confers fitness on a host organism. The biomolecules found in terran life appear to have molecular structures that create properties specifically suited to the demands imposed by water.
• Living systems that have emerged on Earth have done so by a process of random variation in the structure of inherited biomolecules, on which was superimposed natural selection to achieve fitness. These are the central elements of the Darwinian paradigm.